Aluminum and aluminum alloys are traditional metal interconnect metallurgies. While aluminum-based metallurgies have been the material of choice for use as metal interconnects over past years, concern exists as to whether aluminum will meet the demands required as circuit density and speeds for semiconductor devices increase. Because of these concerns, other materials have been researched for use as interconnects in integrated circuits, copper in particular. Advantages of the use of copper as interconnects include a lower susceptibility to electromigration failure as compared to aluminum and a lower resistivity.
A problem with the use of copper as an interconnect metallurgy is that copper readily diffuses into surrounding dielectric materials, especially silicon dioxide. To inhibit this diffusion, copper interconnects are often capped. One method of capping includes the use of a conductive barrier layer along the sidewalls and bottom surface of a copper interconnect. This conductive barrier is typically tantalum or titanium. To cap the upper surface of an interconnect, a dielectric layer, such as silicon nitride is typically used. Due to the need for low temperature processing after the copper is deposited, the silicon nitride layer cannot be deposited at temperatures in excess of 450.degree. C. Accordingly, silicon-nitride deposition is typically performed using plasma enhanced chemical vapor deposition (PECVD) where temperatures generally range from 200.degree. to 425.degree. C. PECVD silicon-nitride has been used in other applications in semiconductor devices. However, in using a silicon nitride cap for copper interconnects, conventional PECVD silicon nitride creates reliability problems. In particular, silicon nitride films deposited using conventional PECVD processes have poor adhesion to copper surfaces. As an example, some nitride films can be peeled from the copper surfaces simply by scratching the film or by removing the film using an adhesive tape. These results are indicative of how the silicon nitride film might adhere to the copper in an actual fabrication process. After being deposited onto the copper surface, additional insulating layers will be deposited over the silicon nitride film. However, subsequent deposition of insulating layers onto the nitride film will produce stresses which can cause the silicon nitride layer to peel from the copper surface. Despite the fact that other layers have been deposited onto the semiconductor device, the fact that the silicon nitride film has peeled from the copper surface, creates a path for copper to diffuse outward and from moisture or other contaminates to diffuse inward.